I2C Pressure Sensor
The BMP085 (Datasheet) from bosch sensortec is a simple, easy to use, pressure sensing device. It speaks I2C and includes a temperature sensor. Because of it's tiny size of 5.0mm x 5.0mm x 1.2mm it fits nice into the Freerunner. I glued mine into the empty space next to the bluetooth antenna.
An experimental kernel driver for the chip is available from gitorious